I. Field of the Invention
The present invention relates generally to a transmitter and to a method of calibrating power in signals output from a transmitter. The invention is useful for controlling the current in a power amplifier in a mobile phone but is not limited to such an application.
II. Description of the Related Art
Until now the emphasis in handset transmitter design for cellular systems has focused on obtaining adequate linearity in the transmitter. The challenge of obtaining high efficiency has only very recently started to be examined. Existing handset designs therefore have very inefficient transmitters. Power control is a significant aspect of the design of cellular based mobile phone systems, particularly, but not exclusively, in systems based on the various CDMA standards that have been developed in recent years and continue to be developed.
Power amplifiers are used in mobile phones to control the power in signals transmitted by the mobile phones. Hitherto such power amplifiers have had either no means to control the bias conditions of the active devices, or some form of very crude means of doing so. Some power amplifiers have been designed to operate in one of two modes, namely a low current mode and a high current mode. The high-current mode is used to obtain adequate linearity at the highest required output powers, and the low-current mode is used to obtain adequate linearity at lower power levels with reduced supply current.
Control circuitry associated with the power amplifier is arranged to generate a control signal for switching the power amplifier between the two modes depending on the required output power. In these amplifier circuits the current control signal is a binary signal that switches operation of the power amplifier between low and high current modes. At lower levels the reduced current drawn from the battery compromises the linearity of the power amplifier. However, at lower output powers there is also less of a requirement for linearity, so the low current mode is used during operation at lower output powers. When the output power from the power amplifier exceeds a certain value (usually between 10 dBm and 20 dBm), the reduced linearity causes the performance to come close to specified limits.
In the case of WCDMA, the parameters of interest are the Adjacent Channel Leakage Ratio (ACLR) and the spectral emission mask. When these values exceed respective predetermined thresholds the power amplifier is switched over to the high current mode, where the linearity is improved at the expense of the higher current drawn from the battery.
While such an arrangement enables the power to be controlled, it does not minimize power consumption during a call. Minimizing power consumption is important in any mobile device because the amount of available battery energy is limited. Hitherto the current control signal has been arranged to change state when it is necessary for the power amplifier to change between the low and high current modes. The actual switching point is usually specified as a certain power amplifier output power. Operation of the power amplifier is at an optimum in terms of linearity, efficiency and power only at maximum power and at the point where the amplifier switches between the low and high current modes.
While the forgoing arrangement is adequate in that it enables accurate power control, it is less than satisfactory in that it does not minimize power consumption by the mobile phone during a call.